The control of the distribution of blood flow to several vessel beds will be studied in representative species of elasmobranchs, teleosts, amphibia and reptiles. Evidence for changes in resistance to blood flow in response to physiological stimuli will be sought by study of the relationship between pressure gradient and blood flow. This study in comparative physiology should give insight into the evolutionary sequence of the development of systems for control of the peripheral circulation. Such information should help in understading perturbations of peripheral vascular control in man.